1. Field of the Invention
This invention relates to a zero-momentum spacecraft and more particularly to a backup reaction wheel to be used with an orthogonal three wheel attitude control system.
2. Description of the Prior Art
Certain earth oriented satellites use a three axis reaction wheel set for attitude control. Increased pointing precision in attitude control can be achieved by means of control torques developed by three reaction wheels in response to sensor-derived attitude error information. The three reaction wheels are usually mounted in the spacecraft in such a manner that their axes are parallel to an orthogonal set of body axes about which attitude control is to be maintained. For an earth-pointing spacecraft, one of the wheel axes is parallel to the spacecraft pitch axis which nominally rotates about the orbit normal at one revolution per orbit. The other two wheel axes are aligned with the spacecraft roll and yaw axes, respectively. In the absence of large external disturbance torques, such a spacecraft system is referred to as a substantially zero-momentum system because the reaction wheel momentum variations about zero would be minimal when the spacecraft products of inertia are small.
A failure of any one of the three orthogonal wheels results in the loss of the satellite's precise attitude control. Many systems have been proposed heretofore for avoiding the problems associated with such a wheel failure. Some prior art attitude control systems use backup wheels on each axis to avoid the problems associated with primary wheel failure. Other prior art attitude control systems use multiple reaction wheels arranged in a non-orthogonal system. Each of the enumerated prior art systems have basic disadvantages in that backup wheels on each axis entail additional cost, weight, and size whereas non-orthogonal arrangements require more complex control laws. In the non-orthogonal arrangement, a wheel has torque components on each of the three orthogonal axes about which attitude control is to be maintained. A reaction wheel which is aligned with an orthogonal axis has no torque components along the other two axes comprising the three axis mutually perpendicular set. Accordingly, good conventional design procedures dictate the use of an orthogonal three axis wheel arrangement to achieve attitude control. It is desirable, therefore, to provide a single backup wheel to be used in place of any one of the three mutually perpendicular reaction wheels.